1. Field of the Invention
This invention lies in the field of processes and apparatus for producing degassed fermentation broth.
2. Prior Art
Fermentors which are pressurizable and which are capable of using high air flow are known, as is technology for culturing microorganisms in such a fermentor. A pressurized culture zone with high air flow is desirable because it improves the rate of oxygen transfer between the microorganisms and the aqueous culturing medium, increases the rate of microorganism growth, and increases the density of microorganisms cultured, but it also increases the foam inherently produced, as those skilled in the art appreciate.
The foam causes a significant problem in post-fermentation separation of the fermentation broth liquid phase from the gas phase which is in the form of entrained gas bubbles (foam). This problem is exacerbated when continuous pressurized fermentation procedures are practiced with high air flow.
The foaming inherently occurs during fermentation, especially when conducted under pressure and high air flow, because the air (oxygen) charged into the fermentor becomes admixed with the aqueous culture medium under the action of medium agitation which is characteristically employed to maintain the medium in a uniform state and to promote transfer of oxygen into microorganisms being aerobically cultured. Certain agents present in the culture medium which originate both from the nutrients charged into the culture medium and from the metabolites excreted from the microorganisms being cultured exert significant surfactant activity and thus tend to create and to stabilize foaming.
Defoaming agents are undesirable additives to a fermentation broth when the fermentation product is intended for food use since they contaminate the broth. Also, the level at which such agents would need to be added in the case of a broth containing a relatively high level of total solids (such as exists when pressurized fermentation is practiced) in order to achieve a practical level of defoaming is so great as to cause operating cost problems and final product purity problems (for example, as regards the level of impurities permitted by the U.S. Food and Drug Administration in human food).
Various mechanical defoaming techniques for use in a fermentor are known, such as rotable metal arm arrangements adapted for flailing the surface regions of a foam, centrifuging, and the like. For example, Hunt U.S. Pat. No. 4,373,024 describes a rotable foam breaking apparatus which is mounted in the upper central portion of a fermentor. When this apparatus functions in an operating fermentor, a foamed fermentation broth is drawn up thereinto along a multitude of paths. At or near the path rotational axes, the gas phase tends to be separated from the liquid phase by centrifugal force, and the latter phase is returned to the main mass of the fermentation broth while the former phase is separated and is vented.
Unfortunately, the rate at which any known apparatus is able to separate the gas phase from the liquid phase in a fermentor seems to be generally insufficient to meet the gas separation requirements existing in high pressure, high air flow continuous fermentation so that such apparatus does not solve the post-fermentation gas separation problem.
In the prior art the post fermentation gas separation problem was typically previously solved by removing the gas-filled fermentation broth from the fermentor and passing such into an open holding reservoir for holding until the gas bubbles collapse. However, the rate of foam collapse is slow, the quantity of gas is large, and the stability of foam is variable from one microorganism to another, and from one nutrient medium to another. Hence, such a "natural" foam removal procedure is not practical for most commercial purposes. Gravity separation is accelerated by the addition of defoaming agent, and the use of such agents in such a separation is sometimes practiced.
Control of the composition of solutes present in an aqueous fermentation liquid (so as thereby to minimize the presence of surface active components therein) is difficult to achieve for many reasons. For one thing, the exact composition of many nutrients and of metabolites from many microorganisms is not now known.
The art of microorganism fermentation, particularly under high pressure conditions, has great need for anew and effective technique for mechanically degassing a foamed fermentation broth.